Semiconductor circuit devices, including integrated circuit devices such as DRAMS and SRAMS, switch internal leads and output drivers on and off as a part of their operation.
Such semiconductor devices typically take the form of a semiconductor die. The die is generally attached to a lead frame within a package, by means of fine gold wires, as shown in FIG. 1. These fine gold wires function as lead frame connection wires. The lead frame and die is then encapsulated, in the form of the familiar integrated circuit "chip". The packaged chip is then able to be installed on a circuit board by any number of techniques, such as socketing and soldering.
The gold connection wires, because of their length relative to their diameter, function as inductors. As current through the gold connection wires is alternately switched on or off, voltage spikes occur. In order to reduce the effects of voltage transients, external capacitors have been installed either within the semiconductor package or on a circuit board onto which the semiconductor packages are installed. In either case, the capacitor is on an opposite side of the lead frame connection wire from the semiconductor die. This establishes the circuit shown in FIG. 2. This equivalent circuit represents an inappropriate arrangement for filtering voltage transients from affecting the box, which represents the semiconductor. Therefore, it is desirable to provide capacitance on the other side of the inductor, i.e., the side of the inductor that the device is connected to.
Most semiconductors, including all DRAMS, include capacitors. For example, a 4 Meg DRAM includes over 4 million capacitors each. For the purpose of storing individual bits of information, these capacitors are accessed by connections through access transistors and sense amps, connected through a periphery circuit. The present invention concerns adding filter capacitance to such devices in order to provide protection from voltage transients which may not be afforded by what may be millions of other capacitors on the semiconductor device.
Semiconductor circuit devices are designed with an architecture which places their functional circuitry within a confined area, usually rectangularly shaped. At the perimeter (either outside or inside) of the rectangularly shaped area are a series of contact pads and a substantial amount of chip area which is occupied by conductor busses, but is unoccupied by active circuit devices. Unlike many of the circuit elements on a semiconductor circuit device, filter capacitors need not be built to precise specifications. It is therefore, possible to utilize perimeter areas and portions of semiconductor chip area which form major border areas between active portions of the semiconductor circuit device.
There is a significant advantage in providing that any added circuit elements be on the same side of a chip wafer as other circuit elements, because of manufacturing techniques and tolerances. Conventionally, semiconductor circuit devices are arrayed on one side of a die wafer. It would therefore be advantageous to design a filtering element which would not significantly expand the die area (chip area) required for each die.
There is a certain portion of the die area which is not particularly suitable for active circuitry. This includes chip area occupied by bus lines, which are normally metallization which overlays most or all of the patterned layers which make up the active circuitry on the die.